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Date: Thu, 30 Jan 2014 16:53:44 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Waiman Long <waiman.long@hp.com>
Cc: Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>, Arnd Bergmann <arnd@arndb.de>,
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        linux-kernel@vger.kernel.org, Steven Rostedt <rostedt@goodmis.org>,
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        Linus Torvalds <torvalds@linux-foundation.org>,
        Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>,
        George Spelvin <linux@horizon.com>,
        Tim Chen <tim.c.chen@linux.intel.com>, aswin@hp.com,
        Scott J Norton <scott.norton@hp.com>
Subject: Re: [PATCH v11 0/4] Introducing a queue read/write lock
 implementation
Message-ID: <20140130155344.GE5002@laptop.programming.kicks-ass.net>
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On Thu, Jan 30, 2014 at 10:50:41AM -0500, Waiman Long wrote:
> One more thing, I often see line like
> 
> #define queue_write_unlock queue_write_unlock
> 
> So exactly what effect does this macro have?

Makes sure the below doesn't emit another version.

#ifndef queue_write_unlock
/**
 * queue_write_unlock - release write lock of a queue rwlock
 * @lock : Pointer to queue rwlock structure
 */
static inline void queue_write_unlock(struct qrwlock *lock)
{
	/*
	 * If the writer field is atomic, it can be cleared directly.
	 * Otherwise, an atomic subtraction will be used to clear it.
	 */
	smp_mb__before_atomic_dec();
	atomic_sub(_QW_LOCKED, &lock->cnts);
}
#endif